NMR Studies of Mononuclear Octahedral Ni(II) Complexes Supported by Tris((2-pyridyl)methyl)amine-Type Ligands

Abstract

The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand (6-Ph₂TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph₂TPA)Ni(CH₃CN)(CH₃OH)](ClO₄)₂ (1), [(6-Ph₂TPA)Ni(ONHC(O)CH₃)]ClO₄ (3), and [(6-Ph₂TPA)Ni−Cl(CH₃CN)]ClO₄ (4), and one Ni(II) complex of tris((2-pyridyl)methyl)amine, [(TPA)Ni(CH₃CN)(H₂O)](ClO₄)₂ (2), have been characterized in acetonitrile solution using conductance methods and NMR spectroscopy. In acetonitrile solution, 1−4 have monomeric cations that exhibit isotropically shifted ¹H NMR resonances. Full assignment of these resonances was achieved using one- and two-dimensional ¹H NMR techniques and ²H NMR of analogues having deuteration of the supporting chelate ligand. COSY cross peaks were observed for pyridyl protons of the 6-Ph₂TPA ligand in 1 and 3. This study lays the groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance to ARD.